Add RemovableMatchChecker

#### Add checker to detect whether match statement is removable or not.

This checker finds removable "match" statements for collections.
[Effective Scala](http://twitter.github.io/effectivescala/) suggests to use pattern matching directly in function definitions if possible.
For example, the checker accepts

    list map {
      case Some(x) => x
      case None => default
    }

but rejects

    list map { item =>
      item match {
        case Some(x) => x
        case None => default
      }
    }

This helps to write pattern matchings clearly.